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Ventilation / heat recovery / energy efficiency

This is a thought process about ventilation and energy efficiency, in ten steps:

1. According to ASHRAE 62.2, a house with 1,500 SF occupied area and two people needs 30 CFM of ventilation air (7.5 CFM per person plus 1 CFM per 100 SF). That’s 1,800 CF of fresh air per hour and 43,200 CF per day. Logically there may be nobody home sometimes and more than two people other times, but let’s consider this the average ventilation volume needed each day for this small household.

2. For optimal energy efficiency, ventilation air volume should not exceed the required 43,200 CF per day. There’s a penalty in space conditioning energy when more outdoor air comes indoors, bypassing the thermal enclosure.

3. Let’s assume that ventilation air volume can be somewhat sporadic (rather than a steady, continuous 30 CFM) without ill effects. This becomes helpful because spot ventilation (on demand) will exceed 30 CFM for a limited time, for example a bath fan running at 50-70 CFM. To avoid over-ventilating, spot ventilation will contribute toward the overall ventilation air volume.

4. Let’s imagine a theoretically optimal exhaust-only setup: bath fans with a combination of demand use and programmed fan cycling that totals close to the ideal 43,200 CF per day. (More on this later.) For now let’s ignore the kitchen range vent. There’s a condensing clothes dryer and no other exhaust devices.

5. A theoretically perfect HRV’s heat recovery core would reduce the delta-T between incoming and outgoing air to zero. If outdoor air is 35°F and indoor is 65°F, delta T is 30°F. Air passing through the core would depart at 50°F both ways. Therefore the perfect HRV saves half the energy lost to ventilation air.

6. The Venmar EKO 1.5, an energy efficient HRV, operates at about 60 CFM at the lowest setting, or 86,400 CF per day. In continuous use this would provide enough ventilation for six people in a house this size.

7. With 60 CFM split among four exhaust registers, each will move 15 CFM. That’s not much for demand exhaust in a bathroom.

8. There could be controls provided in the bathrooms to boost the Venmar to maximum air volume on demand, about 120 CFM. That’s 30 CFM for the bathroom where the boost gets triggered, and the same for the other 3 exhaust locations. If the HRV runs on boost for one hour per day, that adds 3,600 CF to the total ventilation volume (now 90,000 CF).

9. Mechanical ventilation equipment draws electric power. This is another factor in energy efficiency because some devices use considerably more power than others. An efficient exhaust fan (e.g. WhisperGreen series) moves ~10 CFM per watt, whereas the efficient Venmar HRV moves ~2.5 CFM per watt.

10. Even a perfect HRV would break even on space heating energy if it moved double the required air volume, based on point #5. In this example, the Venmar moves double the air volume and uses 5x the electric power per unit of air volume. That is, the HRV uses 10x as much electricity as the exhaust only system in operation.

Now, unless I’ve missed something important, the HRV seems unlikely to come out ahead on energy efficiency. However, it would be possible to program it to run on demand at high speed and remain off much of the time, just like the exhaust only system does here. I would guess the reason the Venmar’s lowest speed is 60 CFM rather than lower (say 30 CFM) is to make it more usable as a bath fan replacement. But even on high speed it’s somewhat ineffective at spot ventilation with 30 CFM per vent (assuming four exhaust locations).

Here’s what I have in mind for an exhaust only system: In each of three bathrooms there’s a fan set to 60 CFM with a motion sensor and 60-minute delay timer. Three fans need to run for 12 hours per day (combined runtime) to meet the air volume target. Using a kitchen range vent (100 CFM on low setting) would reduce this target. At busy times of the day all three bath fans might run at once (moving 180 CFM) while at quiet times they remain still. Ventilation responds to activity, there’s no need to switch fans on/off and they should stay mostly imperceptible (0.3 sones).

The plan for makeup air is a single 6” diameter duct (like an HRV intake) that branches to four supply registers. The intake passes through a filter and a robust gate valve. This valve can be shut for the purposes of blower door testing, which may or may not be cheating. Exhaust ducts can employ redundant backflow dampers (one near the outside of the wall, one near the inside) for the same purpose.

Comments are welcome.

Asked by TJ Elder
Posted Mar 31, 2011 12:13 AM ET


20 Answers

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Your analysis is mostly correct. Ventilation air flows are very low, and it can be hard to justify the cost and electrical energy use of an HRV for a small home. The Panasonic WhisperGreen fans (and the Delta Breez fans) are wonderfully efficient at moving air while using very little electricity. (They just lack heat recovery.)

My reading of the Vernmar Eko 1.5 specs shows that at low speed, the unit moves 49 cfm, not 60 cfm -- a minor point that doesn't much change your basic conclusion.

One point you don't address: designing an effective ventilation system is not just about moving X cubic feet of air in a 24-hour period. It's also about delivering fresh air to a home's occupants, when and where they need it. Obviously, you don't want all of your daily air exchange to happen when everyone in your house takes a shower -- and then to have no ventilation for 23 hours, until it's time for another round of showers.

Answered by Martin Holladay
Posted Mar 31, 2011 4:53 AM ET


Your point 5 says:
"5. A theoretically perfect HRV’s heat recovery core would reduce the delta-T between incoming and outgoing air to zero. If outdoor air is 35°F and indoor is 65°F, delta T is 30°F. Air passing through the core would depart at 50°F both ways. Therefore the perfect HRV saves half the energy lost to ventilation air."

The theoretically perfect HRV, a true counterflow heat exchanger with very large area, and with equal mass flows in and out, would have temperature difference at one or both ends approaching zero. Without any condensation of leaving air, the efficiency would be 100%, not 50%. Many or most HRVs seem to have cross-flow heat exchanger surface units, for various reasons. While this configuration doesn't have the efficiency of a true counterflow exchanger, efficiency reported by the manufacturers typically is in the neighborhood of 70% or more. Some HRVs have a pair of cross-flow exchangers in series, which gives a closer approach to true counterflow.

For example, I looked up two models of the Lifebreath HRV for maximum efficiency. Their model 200MAX has a single core, with a maximum 74% rating. Their 195ECM has a dual core, with a maximum 88% rating

Answered by Dick Russell
Posted Mar 31, 2011 10:45 AM ET


Interesting post Thomas. This is one of those perennial questions.

Seems like you are trading lower capital costs for higher long term operating costs. It's clear that HRV/ERV units cannot match the electrical efficiency of the WhisperGreen and Delta Breez fans, but it's also true that it takes more energy to heat air than move air. So you have to figure in that penalty.

Will the heat recovery savings pay for the higher costing HRV? That's a pretty tricky calculation. Maybe, but it will probably take a long time in the Seattle area.

If you look at the larger picture, there should be some quality benefits to a balanced ventilation system---like having a tempered air supply, and better distribution control. How do these figure into the equation?

Also, controls for an HRV can be more sophisticated---pretty much like a programmable thermostat. Turn the unit off when you are out of town. Program it to run for certain number of minutes per hour. Program it to stop running when you are away at work. Ramp up the speed when you are showering. Ramp up the speed when you have a party and the house is loaded with 15 people. Ramp up the speed when you are in the shower, having a party with 15 people. ;-)

Some people think simple is better. Some like sophistication.

I have a couple of questions on your proposal:

You say three bathrooms, but is this really 2 1/2 bathrooms?

Half-baths do not generate much moisture. If you are looking for odor control (a fart fan in the 1/2 bath), then you could always try these: http://www.courtesyflush.com

Also, if you are going with exhaust only, why bother with make-up air ducts? As long as you test the fans to ensure they are pulling the correct CFM, then what's the benefit? You are going to punch a big hole in your wall, seal and insulate the ducts, then test and balance the very small airflows in each duct branch. Sounds complicated. That, and depressurizing the building in cold weather is really not a problem.

Answered by Daniel Ernst
Posted Mar 31, 2011 11:02 AM ET


While ASHRAE 62.2 may dictate what ventilation is required for code, to ensure adequate fresh air for the house's occupants, you could consider implementing a system that regulates the ventilation based on measured CO2. There are several advantages to this approach.
In our household of 4, plus a medium sized dog, we are gone during the daytime, off to work or school, and come back home on the evenings. On weekends were tend to hangout inside the house, or even have guests over. Obviously, the ventilation requirements change with the number of occupants, so the ability to ramp up or shut off the HRV can save you both electricity and heat loss.
I then placed the HRV on timers, allowing them to run during the daytime only, to take advantage of the higher daytime temps when the air exchange was taking place, therefore reducing heat loss again. I have an override programmed in, so that if the CO2 levels get too high, that the HRV will turn on anyway. I have a data logger for the CO2 and the temperatures in the different room of the house, and have compared how the house has performed before and after adding these tweaks. Most importantly the CO2 levels never get above 900 PPM, even when we had 15 people in the house one evening. Another advantage became apparent as well. Since our house is a bit of a hybrid implementing PassivHaus construction combined with passive solar design, our house can get a bit warm on sunny Winter days. Before the changes it could get close to 80F inside the house....must admit that felt real nice when it was -22F outside! Now the daytime peak temperatures are moderated by the incoming cooler air from the HRV by about 2F, and our morning low inside the house has similiarly gone up by 2F.
Lastly, we have noticed a rather unforeseen advantage to letting the CO2 level ride a bit high at night, our house plants are growing much faster! Kevin

Answered by kevin o'meara
Posted Mar 31, 2011 11:17 AM ET



An easy way to visualize the energy impact of exhaust only ventilation vs heat recovery ventilation is to model your project in PHPP and see how the options impact your annual heat demand. If you have an extremely low energy building you will find that a major portion of your heat demand will be ventilation, even in a mild climate. Without doing the calculations you are shooting in the dark. Additionally there are more benefits to a balanced heat recovery ventilation system than simply less energy consumption.


"Also, if you are going with exhaust only, why bother with make-up air ducts?" If we take a 2000SF home with 8ft ceilings and air seal to Passivhaus standards, simple math tells us that 160cfm of exhaust will depressurize the house to 50 pascals. This is why you need make up air. Pretty simple. And no, building a leaky house and relying on random leaks through flaws in the building envelope is NOT a better solution.

Answered by Skylar Swinford
Posted Mar 31, 2011 11:51 AM ET
Edited Apr 1, 2011 10:35 AM ET.



Your answer is indeed pretty simple, but it assumes that you're wearing a pair of Passivhaus goggles. I don't think Thomas would be proposing an exhaust only ventilation system if he were striving to meet the PH standard. Agreed?

So, assuming a less stringent ACH50 value, say 1.5 - 2.0, what is the benefit to a branched duct system supplying untempered outside make-up air?

Besides the obvious concern over atmospheric combustion appliances, what is the problem with building depressurization in a cold climate?

Answered by Daniel Ernst
Posted Mar 31, 2011 2:33 PM ET


The plan for makeup air includes four registers, with one in each of the two bedrooms. Having ducted makeup air addresses both fresh air and negative pressure. I do believe the combined exhaust fans could create dangerously high negative pressure (280 CFM including the kitchen range). True that winter air has low absolute humidity but in this PNW climate there's a lot of liquid moisture that could get pulled into the walls.

Answered by TJ Elder
Posted Mar 31, 2011 2:43 PM ET


I want to be clear about PH standards / efficiency: I've started thinking recently that this project could in fact meet PH standards, so long as it's sufficiently airtight. Note that I did not state that an exhaust only system was a strategy to cut cost. My analysis suggested that an HRV would use MORE energy.

But maybe I’ve misunderstood the heat exchange mechanism in an HRV. If outgoing air at indoor ambient temperature first passes incoming air that’s almost all the way inside, then at that portion of the air path the delta-T is very small, not nearly the 30 degrees in my example. That’s because the incoming air is not nearly as cold as outdoor ambient, it’s closer to indoor ambient. As outgoing air goes deeper through the core, it gets colder. The temperature of incoming air that it encounters also gets colder. Air leaving the core toward the outdoors should be almost as cold as outdoor ambient and it gets scrubbed of heat by the coldest portion of incoming air. Likewise air heading toward the indoors should be almost as warm as indoor ambient.

If this is closer to how the heat exchanger works, then the theoretical efficiency would approach 100%.

Answered by TJ Elder
Posted Mar 31, 2011 2:49 PM ET


To respond to Martin's comment about how much airflow you get from the Venmar EKO: the manual gives a range that depends on static pressure. This isn't like the motor in a WhisperGreen fan that runs faster in response to greater flow resistance. There's a range of 40-80 CFM (at the lowest speed setting) and I just picked the middle of that range.

If the Venmar HRV actually recovers 74% of the heat from exhaust air, then it only wastes 26% of the heat per unit of air volume, rather than 100% for exhaust-only. Therefore it should break even on space heating energy even when moving 3.85x as much air volume, not at 2x the air volume as in point #10. In the example of 35°F outdoors and 65°F indoors, outgoing air should be 43°F and incoming should be 57°F.

Answered by TJ Elder
Posted Mar 31, 2011 6:38 PM ET



I took a PH project in Portland's climate and added exhaust only ventilation with with 0.1 w/cfm delivery (see attached). I'd say a 115% increase in annual heat demand is substantial.

When you are dealing with a low energy building, ventilation becomes a major piece of the heat load budget. Clearly ventilating without heat recovery breaks the bank...that is if you are trying to meet PH levels of heat demand.

It is important to note that in a PH the heat load is NOT the total energy budget of the building. In this case, the overall energy demand only increased about 10% with the use of exhaust only ventilation. Less extreme, but still considerable.

Don't forget the non-energy benefits of improved control, comfort, and air quality that you will get with a balanced heat recovery ventilation system.

I hope this helps.

0.1 w_CFM Exhaust Ventilation_Portland_OR.jpg 83 ERV_Portland_OR.jpg
Answered by Skylar Swinford
Posted Apr 1, 2011 10:33 AM ET
Edited Apr 1, 2011 10:34 AM ET.


Not to be too contrarian, but am I the only one thinking perhaps the ASHRAE numbers are nuts?

I see similar numbers from the early 70's[slightly different] so I don't think it is a recent tech idea.

Am I crazy to think that building the best available envelope and then leaving the windows open 24/7 is just silly?

I don't think CO2 levels or lack of O2 enter into it

I think perhaps there is a disconnect between the ability to ventilate and the mandate to ventilate.

After accounting for bath and cooking fans which should eliminate the majority of the real danger, moisture, I would think the rest ought to be more occupant controlled.

The idea that we are in some kind of danger if we do not pump our precious heated air out into the 0 degree night strikes me as absurd.

I would think that once a house is "x" months old, most of its outgassing is done, any real 'danger' is gone.

Mind you I am not arguing against the concept of any kind of central ventilation at all, simply the mindset of "Well, the engineer told me to set it here, and gosh that is what it should be.."

I am personally hoping to get my house tight enough to need an HRV, but you can bet I would not be running it at the levels indicated, or all night in the winter.

I believe that the ASHRAE numbers may be important in a large multi unit building, where the windows are likely largely inoperable, and the door may be hundreds of feet from where you sleep. This is where the ability to ventilate and the mandate to ventilate combine. I don't see the same necessity in a single family house.

Answered by Keith Gustafson
Posted Apr 3, 2011 9:19 AM ET


ASHRAE 62.2 tells engineers and builders how to design residential ventilation systems. It says nothing about how residents should operate the systems.

It's prudent for builders to install the correct equipment. But if an occupant chooses to turn off the switch, the builder has no liability.

Answered by Martin Holladay
Posted Apr 3, 2011 3:26 PM ET


Kevin O'Meara,

Thanks for the post. Did you buy the CO2-based system or did you design it yourself?
I'm interested in buying such a control for our remodel project:

I found an excellent article at http://nlcpr.com/OnDemandVentilation.php about CO2-based HRV activation. I spoke with the author and he said:

"Other than the one I made for myself, I am not sure they exist anywhere for
sale. I will make a printed circuit board for it later this year (the
current one I have is hand soldered together using perf-board and the
display is inserted into a box carved out of a piece of leftover 2x4
spruce). When that happens, I can manufacture a bunch of them easily for
anyone that wants one. I am currently gathering data and will probably use
it as an Engineering Masters degree project and work out the math for the
optimal control algorithm.


Answered by Danny Vanderbyl
Posted Apr 4, 2011 11:37 AM ET
Edited Apr 4, 2011 11:38 AM ET.



In your example PHPP model, how many CF of ventilation air did the exhaust-only system move? To be informative for my original thought process, this should be no more than required by ASHRAE 62.2.

Answered by TJ Elder
Posted Apr 5, 2011 8:54 PM ET



In the PHPP model, the exhaust and the ERV moved equal amounts of air (see attached pic). It would make the most sense to model your project in PHPP. This will provide with the data you need to make informed decisions about your design.

With respect to ASHRAE 62.2, it is important to remember it assumes a fairly leaky building shell (at least compared to PH). A post by John Semmelhack back in '09 about ASHRAE's assumed infiltration rate makes this point very well, "... 0.02cfm/ft2 assumed infiltration works out to be roughly 3.0ACH50, FIVE TIMES the level of air-tightness required in a Passive House. At ACTUAL Passive House air changes rates of no more than 0.60ACH50, the assumed infiltration should be changed to 0.004cfm/ft2. Thus, for a theoretical 2,000ft2 Passive House, the ASHRAE 62.2 ventilation rate would be: (0.03-0.004)*2000 + 7.5*4 = 82cfm"

Answered by Skylar Swinford
Posted Apr 8, 2011 4:03 PM ET


Kevin O'meara asked me to comment here. I am the engineer on his project and my company Heliocentric was designer/installer for his systems and controls.

Whether based on ACH (passivhaus method) or on ASHRAE 62.2 (a little more sophisticated) the number will be similar. The main idea is: a ventilation rate per sqft for indoor pollution + a per person ventilation rate.

That said, even with a 95% efficient HRV and ground loop pre-heat - ventilation always losses energy. So It is crucial to make the ventilation demand-based in any moderately efficient building. There are several way to do this: CO2, humidity, and VOC sensors. In practice, a reasonable green home doesn't have high VOCs to worry about, so the others become better indicators of air quality, and other pollutants will get ventilated along with the higher concern key indicators.

CO2 is a very sensitive measure of respiration occurring in the house, and the number of people inside at any given time. It is a direct feedback of our most important ASHRAE variable: people density. There is very good data correlating indoor air quality and CO2 levels.

When you are building anything moderately air tight, and especially passive house levels, when you have a person come home you will see the CO2 levels rise in just minutes. Have a party of 20 people, and you want the HRV to kick onto high, nobody home, don't waste the energy.

When we do passivhaus compact-system style HRV heaters, our controller manages the HRV between CO2 levels for ventilation and temperature for heat demand, depending on the heat requirement state.

Troy Harvey

Answered by ta harvey
Posted Apr 11, 2011 11:19 AM ET



I'm having a trouble with statement 5 in your initial post: "A theoretically perfect HRV’s heat recovery core would reduce the delta-T between incoming and outgoing air to zero. If outdoor air is 35°F and indoor is 65°F, delta T is 30°F. Air passing through the core would depart at 50°F both ways. Therefore the perfect HRV saves half the energy lost to ventilation air. "

This is not an accurate assumption. Once again I'd like to recommend that you utilize PHPP to make energy related design decisions with confidence.

The temperature transfer efficiency of an heat recovery unit can be expressed as (credit Matt Groves with formula):

μt = (t2 - t1) / (t3 - t1)

μt = temperature transfer efficiency
t1 = Outside Intake Temp (temp outside air before the heat exchanger (°F))
t2 =Supply temp (temperature outside (fresh) air after the heat exchanger (°F)
t3 = Room Temp (temp inside air before the heat exchanger (°F))
Note: t3 - t1 is the max temp difference

In your example above, with a 100% efficient HRV, your supply temperature would be 65°F and your exhaust temperature would be 35°F.

I've also attached a spreadsheet that will allow you play around with different recovery efficiencies.

HRV Supply Temps.xlsx 66.38 KB
Answered by Skylar Swinford
Posted Apr 30, 2011 5:11 PM ET


Skylar, I reconsidered the math on that in posts 8 & 9. Maybe I'll edit the original post for posterity.

Answered by TJ Elder
Posted Apr 30, 2011 5:54 PM ET



Indeed you did, sorry I missed it, well maybe the numbers will be useful to someone else troweling the internets for answers.


You stated "So It is crucial to make the ventilation demand-based in any moderately efficient building. There are several way to do this: CO2, humidity, and VOC sensors". Isn't pushing a timed booster switch when you shower or have guests technically "demand-based" ventilation? There is something to be said for simplicity, and thus far this approach has proven to work fantastically in Passivhaus projects across the globe.

The idea behind demand based ventilation is spot on, but I have reliability concerns with the average co2 and humidity sensors on the market and the controls that tie them together. What steps are you taking to commission and maintain/calibrate your "demand" based systems? If you have a preferred sensor that you have had success with I'd love to hear about it. I don't typically like to see my projects looking like an engineer's Christmas tree, but this add-on is simple enough that I'd consider it, especially for a commercial PH.

Answered by Skylar Swinford
Posted May 5, 2011 1:52 AM ET


Honeywell has been making these for years:


But I've never seen one used in a residential application.

Answered by Kevin Dickson, MSME
Posted May 5, 2011 11:23 AM ET

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